When the state transportation commission secures property or property rights required for the construction or reconstruction of the federal aid interstate system, the federal aid primary system or the federal aid secondary system, it is the duty of the district attorney for the county wherein such property or property rights are situate to aid and assist the state transportation commission in all matters pertaining thereto and to be present and assist at any trial involving the property or property rights required upon written request of the secretary or the head of the legal section of the department.